Sacrificial anode and backfill combination

ABSTRACT

An anode and backfill steel protector combination for inserting into at least one cavity which is mechanically formed in steel reinforced concrete. The anode and the backfill steel protector combination comprises an anode for insertion into the at least one cavity with the anode comprising a sacrificial metal element less noble than steel and a backfill which comprises a viscous and pliable ionically conductive material for insertion into the at least one cavity. The backfill is packaged within a cartridge and the cartridge is a cartridge for use with a cartridge which facilitates injecting the backfill from the cartridge into the at least cavity formed in the concrete. The backfill in a cartridge has a shelf life of at least 2 days.

PRIORITY

This application is a Continuation-In-Part and claims priority to patentapplication Ser. No. 12/067,632, filed on Mar. 20, 2008, now U.S. Pat.No. 8,002,964, filed as Application Number PCT/GB2006/050310, on Oct. 2,2006, and to patent application Ser. No. 13/213,151 filed on Aug. 19,2011.

FIELD OF THE INVENTION

The present invention relates to the protection of steel in concreteusing sacrificial anodes, sacrificial anode and backfill combinationssuitable for installation in cavities in concrete, the installation ofsacrificial anodes within the backfills and reinforced concretestructures wherein the steel reinforcement is protected usingsacrificial metal anodes.

BACKGROUND OF THE INVENTION

Discrete sacrificial anodes have been embedded in cavities in concreteto protect the reinforcing steel. In the process the sacrificial metalelement dissolves and a protection current flows from the anode to thesteel. A backfill is a material surrounding the sacrificial metalelement of the anode that maintains an electrolytic contact between theelectrolyte in the surrounding environment and the surface of asacrificial metal element. In anodes for reinforced concrete, thebackfill will also contain an activating agent that maintains anodeactivity. An anode is an electrode that supports a net oxidationreaction on its surface such as the dissolution of a sacrificial metalelement in the case of a sacrificial anode. To protect the steel,electrons must flow from the anode to the steel. This electron movementmay be promoted by the presence of a power supply between the anode andthe steel. The electron movement will primarily occur in electronconducting conductors. Furthermore ions must move through theelectrolyte between the anode and the steel. Positive ions will movefrom the anode to the steel when the steel is protected. A flow of bothelectronic and ionic current occurs in the process of protecting thesteel.

One commercially available sacrificial anode assembly based on WO94/29496 comprises a zinc metal element activated by hydroxyl ions in aporous material that surrounds the zinc. The zinc corrodes to formsoluble products that precipitate out in the pores of the surroundingmaterial. The anode and backfill are pre-formed as a rigid unit. Theunit is subsequently installed in a cavity that is formed in a concretestructure. An embedding mortar, which will typically be a cementitiousrepair mortar that hardens by reaction with water within 12 hours, isused to fill the space between the unit and the concrete surface of thecavity. This mortar fixes the unit in place and provides a path forelectrolytic contact between the electrolyte in the backfill and theelectrolyte in the surrounding concrete.

SUMMARY OF THE INVENTION Technical Problem

The problem to be solved by this invention is to protect reinforcedconcrete using embedded sacrificial metal anodes in an effective andsimple manner.

Sacrificial anodes may be applied to concrete surfaces or embedded incavities formed in the concrete. Anodes applied to concrete surfacesoften loose adhesion to the concrete surface. Embedding compact discretesacrificial anodes in cavities in the concrete provides a solution tothe achievement of a durable attachment between the sacrificial anodeand the concrete. However sacrificial anodes have a limited life becausethey are consumed in the delivery of the protection current and it isdifficult to replace embedded sacrificial anodes at the end of theirlife.

A sacrificial metal element dissolves to form products that often have agreater volume than the metal from which they were derived. As a result,a pressure builds up that can lead to damage in a rigid material likeconcrete. The backfill of a sacrificial anode should be capable ofaccommodating the expansive products of the anodic dissolution reaction.Accommodating the voluminous products of sacrificial metal dissolutionis addressed directly in WO 03/027356 and WO 2005/035831. However, whensacrificial metal dissolution is accelerated using a DC power supply,high volume products will be produced at a rate much greater than thatencountered in the more conventional use of sacrificial anodes. As aresult an improved method of accommodating this relatively rapidexpansion is needed.

Activated sacrificial anode products for embedding in cavities inreinforced concrete normally include a preformed porous solid containingan activating agent around the sacrificial metal element. The anodes arethen embedded in another embedding material in the cavity that is formedin the concrete. As a result at least one additional interface is formedacross which the protection current must flow. These interfaces canpresent weaknesses in the anode system and result from an increase inthe number of processes to form an installed anode system.

The embedding material for discrete sacrificial anodes is normally acementitious mortar that is mixed with water under construction siteconditions and sets as the result of a reaction between the water andthe cement particles. Control of the mixing proportions is moredifficult under site conditions than under factory conditions and itwould be preferable to use and embedding material that does not requirethe mixing of separate components on site.

When a current is impressed off a sacrificial metal element using a DCpower supply, all the metal conductors and connections to thesacrificial metal element are at risk of corroding as these componentsare no longer protected by the natural action of the sacrificial metal.The steel conductor that is normally connected to a sacrificial metalanode would corrode if the sacrificial metal element was driven to asufficiently positive potential by being connected to the positiveterminal of a DC power supply.

Technical Solution

In one example, a method of protecting steel in concrete comprisesforming a cavity in the concrete and placing a puttylike ionicallyconductive backfill in the cavity and inserting a compact discrete anodecomprising a sacrificial metal element less noble than steel into thebackfill such that the sacrificial metal element makes contact with thebackfill and providing a space into which the backfill may move whensubjected to pressure and passing a current from the anode to the steelin the concrete.

In another example, an anode-backfill combination comprises an anodewhich is a compact discrete anode comprising a sacrificial metal elementless noble than steel. The anode is a sacrificial anode for reinforcedconcrete construction. The backfill is packaged in a cartridge-nozzledispenser to ease the installation process on a construction site. Thebackfill is preferably injected from a cartridge-nozzle dispenser into anarrow cavity such as a drilled hole in the concrete structure in asimilar way to that in which sealants like silicone sealant aredispensed. The puttylike ionically conductive backfill is preferablymixed in a factory environment and retains its pliable viscousproperties when it is not exposed to the atmosphere. In this way, nomixing of embedding material components is required on a constructionsite.

The backfill is a pliable, viscous material that may harden slowly toform a weak porous material that can accommodate the corrosion productsproduced by a sacrificial anode. The conductivity of the backfillprimarily arises from one or more dissociated salts within anelectrolyte in the backfill. Possible backfills comprise a colloidalsuspension of fine passive solid particles in water. One example of abackfill consists at least in part of lime putty.

A high current may be induced off the sacrificial metal element to flowto the steel in the concrete for a brief period by, for example,connecting the anode to the positive terminal and the steel to thenegative terminal of a source of DC power. This draws ions such aschlorides and sulphates from the concrete into the backfill. These ionsmay act to maintain sacrificial metal element activity which enables thesacrificial metal element to be connected directly to the steel for usein a more conventional galvanic anode role. The backfill preferablyretains its pliable, viscous properties while a high current density isimpressed off the sacrificial metal element.

The sacrificial metal element is a metal less noble than steel such aszinc, aluminum or magnesium or an alloy thereof. It is preferable toconnect it to a conductor that remains passive when the sacrificialmetal element is connected to the positive terminal of a source of DCpower to form an impressed current anode connection detail. Examples ofpassive conductors include inert conductors like titanium and conductorsthat include an insulating sheath to separate the conductor from theenvironment. The conductor and conductor to anode connection shouldpreferably suffer no more corrosion than a conductor or connection thatremains electrochemically passive while in contact with the electrolytein the concrete and while connected to the anode in the impressedcurrent electrochemical treatment of reinforced concrete.

A space may be provided by venting the backfill to space outside thecavity or by including compressible void space within the cavity orwithin the backfill. The delivery of a high current off the anode mayonly be required for a relatively brief initial period to arrestcorrosion during which a space into which the backfill may move ispreferably provided outside the cavity. A space may also be provided bya weak foamed polymer that traps air within the cavity formed in theconcrete. The foamed polymer is preferably located in close proximity tothe sacrificial metal element.

Advantageous Effects

Embedding compact discrete anodes in cavities in concrete provides amethod of reliably securing the anode to the concrete structure.Impressing a high current density off the anode using a source of DCpower provides a method of rapidly arresting the corrosion process onthe steel. It also draws aggressive ions from the concrete to the anodeto form an activated sacrificial anode and reduces the need to includean activating agent in the backfill.

The provision of a puttylike backfill and a space allows the fastgeneration of high volume product arising from the delivery of a highcurrent density off a sacrificial anode embedded in a cavity in concreteto be accommodated. The putty also retains electrolyte in the longerterm to ensure anode function. The use of a putty as a backfill meansthe interfaces formed may be limited to the interface between thesacrificial metal element and the backfill and the interface between thebackfill and the concrete. The number of processes to achieve aninstalled anode assembly may be reduced as no backfill is applied to thesacrificial metal element in the factory and the backfill installed onsite acts as both a backfill and an embedding material.

The use of a putty as a backfill that retains its pliable viscousproperties when it is not exposed to the atmosphere means that the anodeembedding material can be mixed in a factory environment. It can also bepackaged in cartridge-nozzle dispensers to ease the installation processon a construction site. No mixing of embedding material components isrequired on a construction site. The backfill may be readily injectedfrom a cartridge-nozzle dispenser into a narrow cavity such as a drilledhole in the concrete structure in a similar way to that in whichsealants like silicone sealant are dispensed.

The use of a backfill like lime putty that hardens slowly to form a weakporous material means that the anode can easily be removed at the end ofits useful life and a new anode can be installed in the same hole. Nohard cementitious embedding material needs to be removed.

The overall effect is to make it easier to deliver electrochemicaltreatments to arrest steel corrosion in concrete using sacrificial metalanodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below with reference by way of example to thefigures in which:

FIG. 1 shows a discrete sacrificial metal anode embedded in a puttylikebackfill in a cavity in concrete together with various spaces toaccommodate movement of the backfill.

FIG. 2 shows an anode with strips of compressible foamed polymerattached to the sacrificial metal element.

FIG. 3 shows the current density driven off an aluminum anode embeddedin a lime putty in concrete using a 12V DC power supply connectedbetween the anode and the steel in the concrete.

FIG. 4 shows the galvanic current density off an aluminum anodeconnected to the steel after completing the impressed current treatmentdescribed in FIG. 3.

FIG. 5 shows a schematic representation of a backfill cartridge anddispenser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Brief high current electrochemical treatments have been developed toarrest steel corrosion in concrete. A brief high current treatment mayinvolve the delivery of a charge to the steel of the order of 50 to 500kC/m² (charge per unit area of steel) in a short period. It is possibleto deliver this charge in as little as 48 hours using a power supply butit will typically take 7 days to deliver such a treatment. A sacrificialmetal anode embedded in a cavity in the concrete may be used in animpressed current role to assist in the delivery of a high current for abrief period to arrest corrosion. The current density off thesacrificial metal anode will preferably be greater than 200 mA/m² andmore preferably greater than 1000 mA/m² to deliver this charge in asbrief a period as possible. A high current density will typically bemore than the maximum recommended current driven off anodes of a similartype when used in impressed current cathodic protection. It ispreferable that the brief high current treatment lasts for less than 3months and more preferably less than one month to minimize the time onsite of any temporary equipment that may be used to induce the highcurrent. This high current treatment will preferably be followed by theuse of the same anode in a more conventional galvanic role wherein theanode is connected directly to the steel to prevent corrosion initiationby delivering a small current to the steel. This technology is disclosedin WO 06097770.

During the brief initial high current treatment, a high volume ofproduct arising from sacrificial metal dissolution may be produced.

Accordingly, this invention provides in one aspect, a method ofprotecting steel in concrete that comprises forming a cavity in theconcrete and placing a backfill in the cavity and inserting an anodecomprising a sacrificial metal element less noble than steel into thebackfill and passing a current from the sacrificial metal element to thesteel in the concrete wherein the backfill is an ionically conductivebackfill and the backfill is a pliable viscous material and thedissolution of the sacrificial metal element to produce a high volumeproduct exerts pressure into the pliable viscous backfill and a space ispresent to accommodate backfill movement when the backfill is placedunder pressure.

A brief high current will preferably be induced off the sacrificialmetal element to arrest corrosion on the steel reinforcement. Forexample, this may be achieved by using a temporary source of DC power.It is preferable that the pliable viscous properties of the backfill areat least retained while this high current is delivered from thesacrificial metal element. This will typically be 7 days, but may be asshort as 2 days. It is preferable that the pliable viscous propertiesare retained while the backfill is separated from the atmosphere. Thismay be used to give the backfill a shelf life prior to use. It ispreferable to have a shelf life of at least 1 month and more preferableto have a shelf life of at least 6 months. It is preferable that thebackfill is formed in a factory environment and that the backfill isstored in a cartridge 13. It is preferable that the backfill can beinjected from the cartridge 13 into cavities formed in the concrete.

Accordingly, as shown in FIG. 5, this invention provides in anotheraspect an anode and backfill steel protector combination for insertinginto at least one cavity, hole, or race which is mechanically formed insteel reinforced concrete, the anode and the backfill steel protectorcombination comprising: an anode for insertion into the at least onecavity with the anode comprising a sacrificial metal element less noblethan steel; and a backfill comprising a viscous and pliable ionicallyconductive material for insertion into the at least one cavity; whereinthe backfill is packaged within a cartridge 13 and the cartridge 13 is acartridge 13 for use with a dispenser 14 which facilitates injecting thebackfill from the cartridge 13 into the at least one cavity formed inthe concrete, and the backfill in a cartridge 13 has a shelf life andthe shelf life is at least 2 days.

The backfill in the cartridge 13 has a shelf life that is preferably atleast 7 days and more preferably at least 30 days. A dispenser 14 suchas a cartridge gun is preferably included with the combination.

It is preferable that after exposure to the atmosphere, the backfilldoes not harden fast. More specifically, it is preferable that thecompressive strength of the backfill does not exceed 5 N/mm² within 7days of exposure to the atmosphere and more preferably does not exceed 2N/mm² within 7 days of exposure to the atmosphere. It is preferable thatthe conductivity of the backfill primarily arises from one or moredissociated salts in an electrolyte contained within the backfill.

The anode is preferably sufficiently compact and discrete that it canfit in a hole 50 mm in diameter and 200 mm in length drilled in theconcrete. Alternatively the anode is preferably sufficiently compact anddiscrete that it can fit in a hole or chase 50 mm in depth and 30 mmwide cut in the concrete. It comprises a sacrificial metal element thatis preferably connected to a passive conductor to form an impressedcurrent anode connection. This connection detail means that theconductor and connection remain passive at the positive anode potentialsachieved when the anode is connected to a positive source of DC powerduring an impressed current treatment. More specifically the conductorand connection remain passive at potentials at least as positive as +500mV above the copper/saturated copper sulphate reference electrode andpreferably at least as positive as +2000 mV above the copper/saturatedcopper sulphate reference electrode.

While a high current density is induced off the anode surface, it ispreferable to provide space for backfill movement to outside the cavityand to connect the backfill to this space through an opening. A space toaccommodate backfill movement may also be provided within the cavity inthe concrete. Space may be provided by including a weak compressiblematerial within the cavity or within the backfill or attached to thesacrificial metal element.

In another aspect this invention provides a backfill for use in cavitiesin concrete with a sacrificial metal element less noble than steel toelectrolytically connect the sacrificial metal element to an electrolytein the concrete wherein the backfill substantially comprises asuspension of fine solid particles in water and the fine solid particlesare less than 30 micron in diameter and the conductivity of the backfillprimarily arises from one or more dissociated salts in the watercontained within the backfill and the backfill is a pliable viscousmaterial that retains its pliable viscous properties for a period duringwhich the sacrificial metal element delivers a high current to the steelto arrest steel corrosion and the backfill hardens slowly to form a weakporous material.

The backfill may be supplied as part of an assembly that includes ananode comprising a sacrificial metal element. It is preferable that thesacrificial metal element is connected to a conductor that remainspassive at the positive potentials attained when the anode is used as animpressed current anode to form an impressed current anode connectiondetail. A passive conductor is a conductor that suffers no more anodicdissolution than a conductor that remains electrochemically passive. Theanode may then be connected to the positive terminal of a source of DCpower for a brief period to drive a high current off the sacrificialmetal element to the steel to rapidly arrest steel corrosion.

It is preferable to avoid high water contents in the backfill to limitshrinkage if it dries. More specifically, it is preferable that thewater content of the backfill is less than 60% of the weight of thebackfill.

It is preferable that the fine solid particles are less than 5 micronsin diameter as this eases its installation in narrow cavities and allowsthe backfill to flow smoothly past a sacrificial metal element that isinserted into the backfill in a cavity.

The fine solid particles are preferably passive in water in that they donot substantially react with water. This prevents the loss of thepliable viscous properties of the backfill. More specifically it ispreferable that the backfill remains pliable and viscous for at least 48hours and more preferably for at least 7 days after exposure to theatmosphere so that it may move when the sacrificial metal elementdissolves at a high rate during a brief initial impressed currenttreatment to form high volume corrosion products. Furthermore it ispreferable that the backfill retains its pliable viscous properties fora minimum of 1 month while it is separated from the atmosphere or whileit remains waterlogged. In this way the backfill may be mixed andpackaged in a factory prior to delivery to site. The backfill ispreferably packaged in a cartridge 13 from which it may be directlyinjected into cavities formed in the concrete structure.

The backfill hardens slowly in contact with the atmosphere to form aweak porous material that retains electrolyte and provides space for theformation of voluminous corrosion product arising from the dissolutionof a sacrificial metal element within the backfill at a rate typical ofthat encountered in the galvanic protection of reinforcing steel. It ispreferable that the backfill remains weak to allow easy removal andre-placement of the anode at the end of its functional life. Morespecifically it is preferable that the compressive strength of thebackfill does not exceed 5 N/min² within 7 days of exposure to theatmosphere and more preferably will never exceed 5 N/mm².

It is preferable that a compressible space is provided to accommodatevoluminous corrosion product arising from sacrificial metal dissolution.A compressible element such as a foamed polymer may be included withinthe backfill or within a cavity formed in the concrete or attacheddirectly to the sacrificial metal element. It is preferable that thecompressible element is located adjacent to the sacrificial metalelement to absorb the corrosion products. It is preferable that thecompressible element is attached as strips of compressible material tothe sacrificial metal element of the anode to guide the anode towardsthe center of a cavity that is formed in concrete.

In another aspect this invention provides an assembly comprising anionically conductive backfill and an anode for use in cavities inconcrete wherein the backfill is a pliable viscous material and theanode comprises a compact discrete sacrificial metal element less noblethan steel and a passive conductor wherein the sacrificial metal elementis connected to the passive conductor to form an impressed current anodeconnection characterized in that it suffers no more corrosion than anelectrochemically passive conductor at potentials above thecopper/saturated copper sulphate reference electrode.

An impressed current anode connection detail on a sacrificial metalelement allows the anode to be driven at a high current density withoutcorroding the connection. It is preferable that the passive conductorsuffers no more corrosion than an electrochemically passive conductor atpotentials above +500 mV on the copper/saturated copper sulphatereference electrode scale and more preferably suffers no more corrosionthan an electrochemically passive conductor at potentials above +2000 mVon the copper/saturated copper sulphate reference electrode scale. It ispreferable that the backfill retains its pliable viscous propertieswhile a high current is impressed off the sacrificial metal element tothe steel using a power supply. This impressed current treatment willlast for at least 2 days and will more typically last for at least 7days.

In another aspect this invention provides an ionically conductivebackfill for use in cavities in concrete with a discrete anode lessnoble than steel that substantially comprises lime putty wherein thebackfill is a pliable viscous backfill and the lime putty is a colloidalsuspension of fine calcium hydroxide crystals in water.

In a another aspect this invention provides the production of anassembly to be used in the protection of steel in concrete thatcomprises a sacrificial metal element less noble than steel and apliable viscous backfill that retains its pliable viscous properties forat least 7 days after exposure to the atmosphere and a porous layerwherein the porous layer is formed into a hollow container and thecontainer is at least in part filled with the pliable viscous backfilland the anode is inserted into the backfill.

The assembly is a unit that is separate from the reinforced concrete.Such an assembly may be installed in the large cavities formed whenpatch repairing concrete structures and will be preferred because itlimits the quantity of pliable viscous backfill that would otherwise beused in large cavities that are formed for reasons other than the needto install compact discrete sacrificial anodes in the concrete.

It is preferable that a compressible space is provided within thecontainer and it is preferable that this space is located adjacent tothe sacrificial metal element. It is preferable to seal the container tofacilitate transport and storage of the assembly and if the assembly isto be used in an impressed current mode, it is preferable to provide aportion of the container that is easily broken to create an opening to aspace outside the container.

FIG. 1, shows one example of an anode comprising a sacrificial metalelement 1 less noble than steel that is preferably formed around a wire9 to provide a connection point, located in a cavity 2 in concrete 3 ina pliable viscous backfill 4. The principal anodic reaction is thedissolution of the sacrificial metal. The sacrificial metal element ispreferably selected from aluminum, zinc or magnesium or an alloy ofaluminum, zinc or magnesium.

The anode is preferably a compact discrete anode that may be embedded ina small cavity in the concrete. Examples of such cavities include holesup to 50 mm in diameter and 200 mm in length that may be formed bycoring or drilling as well as longer chases up to 30 mm in width and 50mm in depth that may be cut into the concrete surface. When the cavitiesare holes formed by drilling, it is preferable to keep the diameterbelow 30 mm. A number of anodes will typically be distributed over theconcrete structure to protect the embedded steel. The installation ofanodes in cavities formed in the concrete improves the strength of theattachment or bond between the anode and the concrete and reduces therisk of the anode coming away from the concrete.

The wire 9 is preferably a conductor that remains passive as the anodeis driven to positive potentials to deliver a high current density offits surface.

The sacrificial metal element is substantially surrounded by a pliableviscous backfill 4 and preferably makes direct contact with the pliableviscous backfill. The backfill is not rigid and it is also not a runnyfluid. The properties of the backfill mean that it can move intoadjacent available space when subjected to pressure. The backfillpreferably retains is pliable viscous properties while a high rate ofproduction of voluminous products at the sacrificial metal elementpersists. The rate of production of products is related to the currentdelivered off the sacrificial metal element. High impressed currentdensities may be achieved using a DC power supply and are likely topersist for at least 2 to 3 days although they will more typically bedelivered for one week. High initial current densities may extend to 3months. Thus the backfill should retain its pliable viscous propertiesfor at least 48 hours and will preferably retain these properties for upto 3 months.

The backfill is ionically conductive to support the metal dissolutionreaction on the sacrificial metal element. The conductivity of thebackfill substantially arises from one or more dissociated salts withinan electrolyte contained in the backfill. The resulting ions in theelectrolyte preferably maintain sacrificial anode activity. Examples ofsuch ions include hydroxyl ions, sulphate ions and halide ions. Sulphateand halide ions may be drawn from the surrounding concrete into thebackfill by a brief high impressed current treatment.

The backfill preferably hardens slowly in time, and after hardening itwill preferably form a weak porous material capable of continuing toaccommodate the voluminous products of the anodic reaction that aregenerated at a slower rate. It is preferable that the compressivestrength of the backfill does not exceed 10 N/mm² and more preferablydoes not exceed 2 N/mm².

Examples of the backfill include gels, clays, putty, and retarded cementor fine mortar paste. Cement products harden by reaction with water(hydration). They can therefore harden underwater. In this respect theydiffer from other suggested backfill materials which may only hardenwhen exposed to the atmosphere and, in some cases, the pliable viscousproperties may partially be restored when the backfill is rehydrated.

Gels typically contain more than 60% water. As noted in U.S. Pat. No.6,254,752, a very high water content is an advantage in temporaryelectrochemical treatments designed to deliver high currents for a briefperiod after which the anode system is removed. However dehydration ofthe gel results in shrinkage that can isolate the anode from thesurrounding concrete if the anode is located in a cavity. This is adisadvantage if the anode is intended for longer term use. This will bethe case when the sacrificial metal remaining after an initial impressedcurrent treatment is connected to the steel to provide sacrificial orgalvanic protection.

Improved dimensional stability may be achieved by reducing the watercontent. This may be achieved using a dispersion of fine solid particlesin water. Clay particles are less than 5 microns in diameter and someclays contain less than 50% water when fully saturated. Silt particleswill have diameters of up to 50 microns and sand particles will belarger. The inclusion of larger particles (silt and sand) improvesdimensional stability and results in a courser backfill, but fineparticles are preferred when the backfill is used with a compactdiscrete anode in a small cavity formed in the concrete.

Concretes and mortars include substantial quantities of sand and largeraggregate particles as well as relatively little water. When they arebased on the use of hydraulic cements like Portland cement, the reactionbetween the cement and water will typically produce a rigid material inless than 12 hours. This reaction may be retarded by adding a retardingagent that retards the setting reaction of the cement to retain thepliable viscous properties of the concrete or mortar or cement paste mixfor a longer period. Cement based mixes harden to form relativelyincompressible materials with high compressive strengths which is adisadvantage, but the strength may be reduced and the porosity increasedby increasing the water content of the mix.

A preferred backfill contains lime putty produced by slaking quicklime(CaO) to form a colloidal dispersion of fine calcium hydroxide crystalsin water. Matured lime putty has a relatively consistent volume andreacts with carbon dioxide in the atmosphere to form a weak porousmaterial consisting mainly of calcium carbonate that has a compressivestrength of less than 0.5 N/mm². Lime putty may be blended with othermaterials to improve the properties of the backfill. Lime putty likeclay does not set while it is waterlogged and the puttylikecharacteristics can be partially restored after a short period ofdehydration if it is mixed with water.

A space is provided into which the backfill will move when subjected topressure. The space may be provided outside the cavity through anopening. FIG. 1 shows an example of such an opening 5 connecting thecavity to the external environment. A wide opening from the externalenvironment to the cavity may be partially filled with a sealingmaterial 8 such as a cement or mortar paste, to inhibit rapid moistureloss from the cavity. At the end of a brief high current treatment whenthe formation of voluminous products slows down, it is preferable toseal the opening to the external environment and to use other spacewithin the cavity to accommodate the voluminous products.

A space may be provided by including voids within the backfill 6 orvoids within the cavity 7. The void space may be created using a fillermaterial that traps a compressible fluid like air within the putty orwithin the cavity. An example of a filler material is a weak foamedpolymer such as polystyrene foam.

FIG. 2 shows an example of another anode arrangement where acompressible material such as polystyrene or polyurethane foam 11 isattached to the sacrificial metal element 12 of the anode to form acompressible space. This arrangement has the advantage that thecompressible material is located where it is most needed. The use ofstrips of compressible material can be used to guide the anode into thecenter of the cavity formed in the concrete.

The anode and backfill may be assembled as a separate unit prior toinstallation in a concrete structure. This may be achieved by forming aporous container or mould with an opening to facilitate placing thebackfill and anode in the container. The porous mould or container maybe made using a layer of hydraulic cement or mortar formed into anappropriate shape. Excess water in the cement results in the formationof capillary porosity as the cement hydrates. The mould or container mayalso be formed from a material like cardboard or a porous cloth or evenone or more layers of thin absorbent paper impregnated with a hydrauliccement with a high water to cement ratio. The use of a cement that setsto form a porous material such as hydraulic cement results in a rigidcontainer or mould.

The pliable viscous backfill is installed within the container and theanode is inserted into the backfill. The container then forms an outerporous layer of the anode assembly. A conductor connected to the anodeprotrudes from the container to facilitate making a connection to theanode. The opening to the container may be sealed after the backfill andanode are installed in the container. If an opening venting the backfillto the external environment is left, it is preferable that the containeris a rigid container. If no opening is left, it is preferable that oneor more features is present from the list comprising, a portion of thecontainer or seal is easily broken to create an opening when the anodeis used, a portion of the container or seal is elastomeric, acompressible void space is present in the container. These features areneeded to accommodate expansion within the anode assembly.

Example

An anode 15 mm in diameter and 100 mm long comprising a bar of thealuminum alloy known as US Navy specification MIL-A-24779(SH) that wascast around a titanium wire to facilitate the electrical connection tothe aluminum was embedded in a lime putty in a 25 mm diameter by 130 mmdeep hole in a concrete block. The basic arrangement is shown in FIG. 1.The concrete block measuring 380 by 270 by 220 mm was made using gradedall-in-one 20 mm aggregate and ordinary Portland cement in the ratio8:1. The water to cement ratio was 0.6 and 4% chloride ion by weight ofcement was added to the mix by dissolving sodium chloride in the mixwater. A sheet of steel with a surface area of 0.125 m² was included inthe concrete block. The lime putty was produced by slaking and maturingquicklime and was sourced from a manufacturer of lime putty and limemortars. The hole in the concrete block containing the lime putty andthe anode was left open to the air. The concrete block was stored in adry indoor environment and the temperature varied between 10 and 20 C.

The anode and the steel were connected to a 12 Volt DC power supply fora period of 13 days during which a charge of 65 kC was delivered fromthe anode to the steel. The current density delivered off the anode forthe first 11 days is given in FIG. 2. For most of this time, the currentdelivered off the anode was greater than 5000 mA/m². During this periodwater and corrosion products accumulated at the location of the anodeand moved out of the hole containing the anode and the putty onto thesurface of the concrete.

At the end of the period of impressed current treatment, the DC supplywas removed and the anode was connected to the steel. The galvaniccurrent out of the anode was measured using a 1 ohm resistor as acurrent sensor in the connection between the anode and the steel. Thecurrent density delivered off the anode acting purely in a galvanic modefor the next 40 days is given in FIG. 3. For most of this period, thecurrent density delivered off the anode was between 500 and 600 mA/m².This indicates that a high degree of anode activation was achieved bydrawing chloride from the concrete into the backfill around thesacrificial metal element during the impressed current treatment.

We claim:
 1. An anode and backfill steel protector combination forinserting into at least one cavity which is mechanically formed in steelreinforced concrete, the anode and the backfill steel protectorcombination comprising: an anode for insertion into the at least onecavity with the anode comprising a sacrificial metal element less noblethan steel; and a backfill comprising a viscous and pliable ionicallyconductive material for insertion into the at least one cavity; whereinthe backfill is packaged within a cartridge and the cartridge is acartridge for use with a cartridge which facilitates injecting thebackfill from the cartridge into the at least cavity formed in theconcrete, and the backfill in a cartridge has a shelf life and the shelflife is at least 2 days.
 2. The anode and backfill steel protectorcombination as claimed in claim 1 wherein the shelf life is at least 7days.
 3. The anode and backfill steel protector combination as claimedin claim 1 wherein the shelf life is at least 30 days.
 4. The anode andbackfill steel protector combination as claimed in claim 1 wherein adispenser is included with the combination.
 5. The anode and backfillsteel protector combination as claimed in claim 4 wherein the dispenseris a cartridge gun.
 6. The anode and backfill steel protectorcombination as claimed in claim 1 wherein the anode is sized to fit intoa cavity which a hole is 50 mm or less in diameter and is 200 mm or lessin length.
 7. The anode and backfill steel protector combination asclaimed in claim 1 wherein the anode is sized to fit into a cavity whichis a chase 50 mm or less in depth and 30 mm or less in width.